Constant velocity joint

ABSTRACT

A constant velocity joint includes an outer race having a plurality of ball grooves formed in an inner spherical surface thereof, which ball grooves extend in a direction of a rotation axis of the outer race, an inner once having a plurality of ball grooves formed in an outer spherical surface thereof, which ball grooves extend in a direction of a rotation axis of the inner race and are paired with the ball grooves of the outer race, and a plurality of balls disposed between the ball grooves of the outer race and the ball grooves of the inner race. In the constant velicity joint, the ball grooves of at least one of the outer race and the inner race are partially formed with relief profiles to provide a plurality of relief groove portions such that a load applied to each of the balls located in the relief groove portions during torque transmission between the inner race and the outer race is smaller than a load applied to each of the balls located in the other portions of the ball grooves.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to a constant velocity joint, and moreparticularly to a ball-type constant velocity joint having a pluralityof balls captured between an inner race and an outer race of the joint.

2. Description of Related Art

In front-wheel drive vehicles and four-wheel drive vehicles, forexample, constant velocity joints are generally used in couplingportions between front wheels serving as steering/driving wheels anddrive shafts. Such a constant velocity joint allows changes in the jointangle between a driving shaft and a driven shaft connected to the joint,while permitting the driving shaft and the driven shaft to rotate at anequal angular velocity. As one type of the constant velocity joints, aball-type constant velocity joint is known in which a plurality of ballsare captured between an inner race and an outer race of the joint suchthat torque is transmitted through contact points of the balls withinner walls of grooves formed in the inner and outer races.

The inner race of the ball-type constant velocity joint has an outerspherical surface of a generally spherical shape, and is accommodated inthe outer race having an inner spherical surface of a generallyspherical shape. In the constant velocity joint, ball grooves extendingin the direction of the rotation axis of the joint are formed in theouter spherical surface of the inner race and the inner sphericalsurface of the outer race at equal intervals in the circumferentialdirection. The number of the ball grooves formed in each of the innerand outer races is the same as that of the balls. The balls arerespectively disposed between the opposed ball grooves of the inner raceand the outer race, such that the intervals between the balls asmeasured in the circumferential direction are fixed or maintained by acage that is interposed between the outer circumferential surface of theinner race and the inner circumferential surface of the outer race.

With the ball-type constant velocity joint thus constructed, the anglebetween the rotation axis of the outer race and the rotation axis of theinner race can freely change through movements of the balls along theball grooves. On the other hand, the displacement of the balls in thecircumferential direction is restrained or inhibited by side walls ofthe ball grooves in the outer spherical surface of the inner race andthe inner spherical surface of the outer race, so that relative rotationof the inner race and the outer race is inhibited. With thisarrangement, the inner race and the outer race are able to rotate at anequal angular velocity, while permitting changes in the joint anglebetween the inner and outer races.

As one type of the ball-type constant velocity joint as described above,a so-called UF (undercut free) type constant velocity joint is known inwhich ball grooves in an open end portion of the outer race are shapedin the form of straight grooves that extend in parallel with therotation axis of the outer race.

In the UF-type constant velocity joint, the outer race can be formedwith its opening having an increased inside diameter without increasingthe outside diameter of the outer race. With the outer race thus formed,the joint angle is less likely to be restricted or limited byinterference between a shaft connected to the inner race and the inneredge of the open end portion of the outer race, and the joint angle canbe accordingly increased.

Japanese Laid-open Patent Publication No. 2001-153149 discloses anotherexample of the UF type constant velocity joint in which the maximumjoint angle can be further increased by forming the straight grooves ofthe outer race so that each groove extends straight in such a directionthat the distance between the bottom of the groove and the rotation axisof the outer race increases toward the open end thereof.

However, the known UF-type constant velocity joints may suffer from thefollowing problems caused by the increase in the maximum joint angle.

The outer spherical surface of the inner race and the inner sphericalsurface of the outer race are formed in substantially uniform arcuateshape as viewed in a plane that contains the rotation axes thereof. Ifthe ball grooves are formed to extend substantially straight in theouter race, the depth of the ball grooves cannot be made constant, andsome portions of the ball grooves inevitably have a reduced depth.

Also, the displacement of the balls in the radial directions duringrotation of the joint increases as the maximum joint angle increases,and therefore the outside diameter of the cage holding the balls tendsto be increased. The increase in the cage diameter requires the innerspherical surface of the outer race to be enlarged, resulting in areduction in the groove depth of the ball grooves of the outer race as awhole.

In the constant velocity joint as described above, torque is transmittedbetween the outer race and the inner race via the respective balls towhich a load or torque is distributed. It is thus difficult to ensuresufficiently high durability at portions of the outer race in which theball grooves have a relatively small depth as described above. To ensuresufficiently high durability at these portions, the depth of the overallball grooves needs to be increased, and the ball diameter also needs tobe increased. The increases in the groove depth and ball diameterinevitably result in increases in the size and weight of the constantvelocity joint.

The constant-velocity joint as disclosed in the above-identifiedpublication is likely to suffer from the above-described tendenciesbecause of the configuration of the ball grooves.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a constantvelocity joint having an improved durability without increasing the sizeor weight thereof.

To accomplish the above and/or further object(s), there is providedaccording to one aspect of the invention a constant velocity jointincluding (a) an outer race having a plurality of ball grooves formed inan inner spherical surface thereof, the ball grooves extending in adirection of a rotation axis of the outer race, (b) an inner race havinga plurality of ball grooves formed in an outer spherical surfacethereof, the ball grooves extending in a direction of a rotation axis ofthe inner race and being paired with the ball grooves of the outer race,and (c) a plurality of balls disposed between the ball grooves of theouter race and the ball grooves of the inner race, wherein the ballgrooves of at least one of the outer race and the inner race arepartially formed with escape or relief profiles to provide a pluralityof relief groove portions such that a load applied to each of the ballslocated in the relief groove portions during torque transmission betweenthe inner race and the outer race is smaller than a load applied to eachof the balls located in the other portions of the ball grooves.

Upon rotation of the constant velocity joint constructed as describedabove, each of the balls interposed between the ball grooves formed inan outer spherical surface of the inner race and the ball grooves formedin an inner spherical surface of the outer race functions to transmit aload therethrough so as to perform torque transmission between the innerrace and the outer race.

With the ball grooves partially formed with escape or relief profiles asdescribed above, part of the balls move into the relief groove portionsduring rotation of the joint, depending upon an angle (i.e., jointangle) formed by the rotation axis of the inner race and that of theouter race. In this condition, a load applied to each of the ballslocated in the relief groove portions during torque transmission issmaller than a load applied to the balls located in the other portionsof the ball grooves.

Accordingly, the relief groove portions are required to have a smallermechanical strength than the other portions of the ball grooves, andthus assures sufficiently high durability even if they have reducedgroove depths. By forming parts of the ball grooves having reduceddepths with relief profiles to provide the relief groove portions,therefore, the small-depth parts of the ball grooves still exhibitsufficiently high durability.

Furthermore, the reduction in the required groove depth in the reliefgroove portions leads to an enhanced freedom in the design of the jointin terms of, for example, the radius of curvature of the ball groovesand the ball size, and also leads to reductions in the size and weightof the constant velocity joint.

According to another aspect of the invention, there is provided aconstant velocity joint including (a) an outer race having a pluralityof ball grooves formed in an inner spherical surface thereof, the ballgrooves extending in a direction of a rotation axis of the outer race,(b) an inner race having a plurality of ball grooves formed in an outerspherical surface thereof, the ball grooves extending in a direction ofa rotation axis of the inner race and being paired with the ball groovesof the outer race, and (c) a plurality of balls disposed between theball grooves of the outer race and the ball grooves of the inner race,wherein the ball grooves of at least one of the outer race and the innerrace are partially formed with relief profiles to provide a plurality ofrelief groove portions such that a clearance between each of the ballslocated in the relief groove portions and inner walls of the ballgrooves as viewed in a circumferential direction is larger than aclearance between each of the balls located in the other portions of theball grooves and the inner walls of the ball grooves.

In the constant velocity joint constructed as described above, a loadapplied to each of the balls located in the relief groove portionsduring torque transmission can be made smaller than a load applied tothe balls located in the other portions of the ball grooves, thusassuring sufficiently high durability even if the relief groove portionshave reduced depths Furthermore, the reduction in the required groovedepth in the relief groove portions leads to an enhanced freedom in thedesign of the joint in terms of, for example, the radius of curvature ofthe ball grooves and the ball size, and also leads to reductions in thesize and weight of the constant velocity joint.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and/or further objects, features and advantages of theinvention will become more apparent from the following description ofpreferred embodiments with reference to the accompanying drawings, inwhich like numerals are used to represent like elements and wherein:

FIG. 1 is a cross-sectional view showing a longitudinal cross section ofa constant velocity joint according to one embodiment of the invention;

FIG. 2 is a cross-sectional view showing a part of a longitudinal crosssection of an outer race of the constant velocity joint of FIG. 1;

FIG. 3 is a cross-sectional view showing a part of a longitudinal crosssection of an inner race of the constant velocity joint of FIG. 1;

FIG. 4 is a cross-sectional view showing a part of a longitudinal crosssection of the outer race;

FIG. 5 is a cross-sectional view showing a part of a longitudinal crosssection of the inner race;

FIGS. 6A-6C are schematic views showing examples of engagement between aball and ball grooves;

FIG. 7 is a graph indicating the relationship between the ball positionand the contact angle at which the ball contacts the side walls of thecorresponding ball grooves;

FIG. 8 is a graph showing the relationship between the ball position andthe PCR clearance; and

FIG. 9A-9D are graphs indicating the relationship between the rotationalphase of the constant velocity joint and the position of each ball.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One exemplary embodiment of the invention will be described in detailwith reference to the accompanying drawings.

A constant velocity joint 10 according to the embodiment of theinvention as shown in FIG. 1 generally includes an inner race 11connected to an end portion of a driving shaft (not shown), and an outerrace 13 connected integrally to a driven shaft 12. The constant velocityjoint 10 further includes a plurality of balls 14 (i.e., six balls inthis embodiment) that are interposed between the inner race 11 and theouter race 13 and are arranged to transmit torque between the inner andouter races 11, 13, and a cage 15 for holding these balls 14. FIG. 1shows a state of the constant velocity joint 10 in which the joint angleθ is equal to the maximum angle θmax.

A through-hole 16 is formed in the inner race 11 to extend through theinner race 11 in the direction of its rotation axis L1, as shown inFIG. 1. A spline 16 a extending in the direction of the rotation axis L1is formed in the inner circumferential wall of the through-hole 16. Byfitting the end portion of the driving shaft into the through-hole 16,the inner race 11 and the driving shaft are connected to each other suchthat the inner race 11 is rotatable as a unit with the driving shaft.

An outer spherical surface 17 having a generally spherical shape isformed on the outer periphery of the inner race 11. A plurality of ballgrooves 18 (i.e., six grooves in this embodiment) whose number is thesame as that of the balls 14 are formed in the outer spherical surface17, such that the ball grooves 18 are spaced from each other at equalintervals about the rotation axis L1. Each of the ball grooves 18extends in the direction of the rotation axis L1 of the inner race 11.

On the other hand, the outer race 13 has an opening 19 formed at adistal end thereof (on the right-hand side in FIG. 1) such that a cavity20 that receives the inner race 11 is formed in the outer race 13. Thecavity 20 of the outer race 13 is defined by an inner spherical surface21 having a generally spherical shape. A plurality of ball grooves 22(i.e., six grooves in this embodiment) whose number is the same as thatof the balls 14 are formed in the inner spherical surface 21 of theouter race 13, such that the ball grooves 22 are spaced at equalintervals about the rotation axis L2 of the outer race 13, and arerespectively opposed to the ball grooves 48 of the inner race 11. Eachof the ball grooves 22 extends in the direction of the rotation axis L2of the outer race 13.

The cage 15 as indicated above is disposed between the outer sphericalsurface 17 of the inner race 11 and the inner spherical surface 21 ofthe outer race 13. The cage 15 is formed with a plurality of ballholding windows 23 (six windows in this embodiment) whose number is thesame as that of the balls 14, such that the ball holding windows 23 arespaced from each other at equal intervals in the circumferentialdirection thereof. Each of the balls 14 is interposed between thecorresponding ball grooves 18, 20 of the inner and outer races 11, 13while being received or held in a corresponding one of the ball holdingwindows 23 of the cage 15.

Next, the profiles of the ball grooves 18, 22 of the constant velocityjoint 10 according to the present embodiment will be described referringto FIG. 2 and FIG. 3.

FIG. 2 shows a cross section of the outer race 13 as viewed from oneside thereof, and FIG. 3 shows a cross section of the inner race 11 asviewed from one side thereof. A curve L3 shown in FIG. 2 is a trajectoryof the center of curvature of the side walls of each ball groove 22taken in a plane perpendicular to the direction of the rotation axis L2in which the ball grooves 22 extend. Similarly, a curve L4 as shown inFIG. 3 is a trajectory of the center of curvature of the side walls ofeach ball groove 18 taken in a plane perpendicular to the direction ofthe rotation axis L1 in which the ball grooves 18 extend. Namely, thecurves L3, L4 show a trajectory of the center of the ball 14 that isguided along the respective ball grooves 18, 22. In the followingdescription, the curve L3, L4 will be called “center-of-curvature line”of each ball groove 18, 22.

In this specification, the end portion (on the right-hand side of FIG. 1and FIG. 2) of the outer race 13 (as viewed in the direction of therotation axis L2) in which the opening 19 is formed will be referred toas “open end portion” or “open end”, and the opposite end portion (onthe left-hand side of FIG. 1 and FIG. 2) of the outer race 13 oppositeto the open end portion will be referred to as “inner end portion” or“innermost side”. Similarly, the right-hand side portion of the innerrace 11 as viewed in the direction of the rotation axis L1 in FIG. 1will be referred to as “open end portion” or “open end”, and theleft-hand side portion thereof as viewed in FIG. 1 will be referred toas “inner end portion” or “innermost side”.

As shown in FIG. 2, each ball groove 22 of the outer race 13 includes astraight groove portion 22 a formed at the open end portion thereof, andan arcuate groove portion 22 b formed on the inner side thereof. In thestraight groove portion 22 a, the ball groove 22 extends in a directionparallel to the rotation axis L2 of the outer race 13 in a plane thatextends along the rotation axis L2.

In the arcuate groove portion 22 b, the ball groove 22 extends along anarc having a center of curvature O2 on the rotation axis L2. As shown inFIG. 2, the center of curvature O2 of the arcuate groove portion 22 b islocated on the rotation axis L2 at a position that is offset by apredetermined length toward the open end from the center of curvature O3of the inner spherical surface 21 of the outer race 13.

A distance from the center of curvature O2 to the center-of-curvatureline L3 in the arcuate groove portion 22 b and a distance from therotation axis L2 to the center-of-curvature line L3 in the straightgroove portion 22 a will be called “radius of curvature (PCR) of theouter race ball groove 22”.

As shown in FIG. 3, each ball groove 18 of the outer spherical surface17 of the inner race 11 includes a straight groove portion 18 a formedon the inner side, and an arcuate groove portion 18 b formed at the openend portion. In the straight groove portion 18 a, the ball groove 18extends in a direction parallel to the rotation axis L1 of the innerrace 11 in a plane that extends along the rotation axis L1. In thearcuate groove portion 18 b, the ball groove 18 extends along an archaving a center of curvature O1 on the rotation axis L1. As shown inFIG. 3, the center of curvature O1 of the arcuate groove portion 18 b islocated on the rotation axis L1 at a position offset by a predeterminedlength toward the innermost side from the center of curvature O4 of theouter spherical surface 17 of the inner race 11.

A distance from the center of curvature O4 to the center-of-curvatureline L4 in the accuate or arc groove portion 18 b and a distance fromthe rotation axis L1 to the center-of-curvature line L4 in the straightgroove portion 18 a will be called “radius of curvature (PCR) of theinner race ball groove 18.

The radius of curvature PCR of the inner race ball groove 18 is madeslightly smaller than that (PCR) of the outer race ball groove 22 so asto permit smooth movements of the ball 14 between the ball grooves 18,22. Namely, the ball 14 is disposed between the ball grooves 18, 22 witha slight clearance left in the radial direction. A difference betweenthe PCR of the inner race ball groove 18 and the PCR of the outer raceball groove 22 is called “PCR clearance”.

Thus, the constant velocity joint 10 of this embodiment is constructedas the UF-type constant velocity joint as described above in which theopen end portion of each ball groove 22 formed in the inner sphericalsurface 21 of the outer race 13 provides a straight groove that extendsin parallel with the rotation axis L2.

In the constant velocity joint 10 constructed as described above, eachball 14 is moved along the corresponding ball grooves 18, 22 so that therotation axis L1 of the inner race 11 tilts about a fixed center O,relative to the rotation axis L2 of the outer race 13.

In operation, torque is transmitted through the balls 14 interposedbetween the respective ball grooves 18, 22, so that the driven shaft 12connected to the outer race 13 is rotated in accordance with rotation ofthe driving shaft connected to the inner race 11. At this time, the sidewalls of the ball grooves 18, 22 restrict displacement of each ball 14in the circumferential direction of the inner race 11 or the outer race13, thereby inhibiting the inner race 11 and the outer race 13 fromrotating relative to each other. With this arrangement, the inner race11 and the outer race 13 can be rotated at an equal angular velocitywhile at the same time permitting changes of the joint angle θ.

As shown in FIG. 2, the center of curvature O2 of the arcuate grooveportion 22 b is offset from the center of curvature O3 of the innerspherical surface 21 in the outer race 13, and therefore the depth ofthe groove of the arcuate groove portion 22 b decreases toward theinnermost side (i.e., left-hand side in FIG. 2). Also, as shown in FIG.3, the depth of the straight groove portion 18 a of the inner race 11decreases toward the innermost side (i.e., left-hand side in FIG. 3).

In the constant velocity joint 10 of this embodiment, portions (18 c, 22c) of the ball grooves 18, 22 having a decreasing groove depth areformed as “relief groove portions”. Cross sections of the relief grooveportions 18 c, 22 c taken in a direction perpendicular to thecenter-of-curvature lines L3, IA, respectively, are formed with reliefprofiles such that clearances between the balls 14 and the side walls ofthe ball grooves 18, 22 as measured in the circumferential direction ofthe inner race 11 or the outer race 13 in the relief groove portions 18c, 22 c are made larger than those in the other portions of the ballgrooves 18, 22.

Here, the position of each ball groove 18, 22 as viewed in thelongitudinal direction is represented by a groove-section joint angle φ,which is defined as follows.

FIG. 4 shows a cross section of the outer race 13 taken in a plane thatcontains both the center-of-curvature line L3 of a certain ball groove22 of the outer race 13 and the rotation axis L2 of the outer race 13. Astraight line L5 as shown in FIG. 4 indicates the rotation axis L1 ofthe inner race 11 when the ball 14 is located on the ball groove 22 atthe position of FIG. 4 where the joint angle is taken in theabove-described cross section.

Here, an angle φ of inclination of the straight line L5 with respect tothe rotation axis L2 in the counter clockwise direction is called“groove-section joint angle” associated with the outer race 13. When thegroove-section joint angle φ is equal to a certain angle φ1, theposition of the ball groove 22 at which the ball 14 contacts the sidewalls of the groove 22 is called “p1 position”, which indicates that thegroove-section joint angle φ established with the ball 14 located inthis position of the ball groove 22 is equal to 1.

Similarly, the position of the ball groove 18 of the inner race 11 isrepresented by the groove-section joint angle φ. FIG. 5 shows a crosssection of the inner race 11 taken in a plane that contains both thecenter-of-curvature line L4 of a certain ball groove 18 of the innerrace 11 and the rotation axis L1 of the inner race 11. A straight lineL6 as shown in FIG. 5 indicates the rotation axis L2 of the outer race13 when the ball 14 is located on the ball groove 18 at the position ofFIG. 5 where the joint angle is taken in the above-described crosssection.

Here, an angle φ of inclination of the straight line L6 with respect tothe rotation axis L1 in the clockwise direction is called“groove-section joint angle” associated with the inner race 11. When thegroove-section joint angle φ is equal to a certain angle φ2, theposition of the ball groove 18 at which the ball 14 contacts the sidewalls of the groove 18 is called “φ2 position”, which indicates that thegroove-section joint angle φ established with the ball 14 located inthis position of the ball groove 18 is equal to φ2.

The definition of the groove-section angle φ has been described above.In the present embodiment, the range of the relief groove portion 18 c,22 c in each of the ball grooves 18, 22 is set and expressed in themanner as described below, using the groove-section joint angle φ.

More specifically, a portion of each ball groove 22 of the outer race 13in which the groove-section joint angle φ is equal to or larger than apredetermined angle φa (=γ2) is set as the above-indicated relief grooveportion 22 c. Also, a portion of each ball groove 18 of the inner race11 in which the groove-section joint angle φ is equal to or smaller thana predetermined angle −φa (=γ1) is set as the relief groove portion 18c.

In the constant velocity joint 10 as described above, when the jointangle θ is equal to 0°, each ball 14 is held at a position where thegroove-section joint angle φ of the ball grooves 18, 22 is equal to 0°,irrespective of the rotational phase of the inner race 11 and the outerrace 13. In this condition, torque transmission between the inner race11 and the outer race 13 is carried out with the torque substantiallyequally distributed to all of the balls 14. Namely, substantially thesame portion of torque is applied to each ball 14, to perform torquetransmission between the inner race 11 and the outer race 13.

When the rotation axis L2 of the outer race 13 is inclined relative torotation axis L1 of the inner race 11, each ball 14 reciprocates alongthe corresponding ball grooves 18, 22 in accordance with changes in therotational phase of the inner race 11 and the outer race 13. When thejoint angle θ is equal to θ1 (0°<θ1≦θmax), the ball 14 moves in thecorresponding ball grooves 18, 22 within a range from a position atwhich the groove-section joint angle φ is equal to −θ1 to a position atwhich the groove-section joint angle φ is equal to θ1. As the jointangle θ increases, the range of movement of the ball 14 in the ballgrooves 18, 22 increases both on the side of the open end and the innerside with its center set at a position where the groove-section jointangle φ is equal to 0°.

When the joint angle θ is less than the above-indicated angle φa, all ofthe balls 14 are located at portions of the ball grooves 18, 22 otherthan the relief groove portions 18 c, 22 c, irrespective of therotational phase of the inner race 11 and the outer race 13. In thiscondition, torque transmission between the inner race 11 and the outerrace 13 is performed with the torque distributed to all of the balls 14.

When the joint angle θ is equal to or greater than the above-indicatedangle ha, each ball 14 enters the relief groove portions 18 c, 22 c ofthe corresponding ball grooves 18, 22, depending upon the rotationalphase of the inner race 11 and the outer race 13.

In this condition, a load applied to each of the balls 14 for torquetransmission is reduced when the ball 14 is located in the ranges of therelief groove portions 18 c, 22 c having increased clearances betweenthe ball 14 and the side walls of the ball grooves 18, 22, as comparedwith a load applied to each of the balls 14 located outside of theranges of the relief groove portions 18 c, 22 c. In this case, thereduced amount of torque is distributed to the balls 14 located outsideof the ranges of the relief groove portions 18 c, 22 c.

Thus, in the present embodiment, the portions of the ball grooves 18, 22having relatively small groove depths are formed with relief profiles asdescribed below, and therefore load (torque) transmission through theballs 14 located in the relief regions can be reduced or suppressed.

Next, the cross-sectional shapes of the relief groove portions 18 c, 22c of the ball grooves 18, 22 will be described in detail with referenceto FIG. 6A and FIG. 6B.

FIG. 6A shows a ball 14 which engages with portions of the ball grooves18, 22 other than the relief groove portions 18 c, 22 c. It is to benoted that in FIG. 6A through 6C, clearances between the side walls ofthe ball grooves 18, 22 and the ball 14 are emphasized.

As shown in FIG. 6A, ball contact angles (which will be described later)at which the ball 14 contacts with the side walls of the ball grooves18, 22 are all set to a predetermined angle α over the entire regions ofthe portions of the ball grooves 18, 22 other than the relief grooveportions 18 c, 22 c. In these portions of the ball grooves 18, 22, thePCR clearance as indicated above is set to a predetermined length CL1.

The ball contact angle means an angle formed between line La and line Lbas follows. The line La is a centerline as viewed in the circumferentialdirection of the ball groove 18, 22 in a plane perpendicular to thecenter-of-curvature line L4, L3. More specifically, the line La is astraight line that connects the center-of-curvature line L4 of the ballgroove 18 with the rotation axis L1 of the inner race 11 in theabove-indicated plane, or a straight line that connects thecenter-of-curvature line L3 of the ball groove 22 with the rotation axisL2 of the outer race 13. The line Lb is a straight line that connects acontact point C at which the ball 14 contacts with each side wall of theball groove (18, 22) with the center of the ball 14.

In the relief groove portion 22 c, on the other hand, thecross-sectional shape of the ball groove 22 is formed such that thecontact point C as indicated above is located closer to the center ofthe ball groove 22, namely, the ball contact angle is equal to angle βthat is smaller than the above-indicated angle α as shain in FIG. 6BWith the relief groove portion 22 c thus formed, clearances between theball 14 and the side walls of the ball groove 22 in the relief grooveportion 22 c are expanded or increased. Similarly, in the relief grooveportion 18 c of the inner race 11, the cross-sectional shape of the ballgroove 18 is formed such that the ball contact angle becomes smallerthan the above-indicated angle α.

In the present embodiment, the ball contact angle in the ball groove 22of the outer race 13, which is equal to the angle cc in the portionsother than the relief groove portion 22 c, gradually decreases from theinitial position of the relief groove portion 22 c, namely, the positionat which the groove-section joint angle α is equal to φa (=γ2), towardthe innermost side as shown in FIG. 7. Similarly, the ball contact anglein the ball groove 18 of the inner race 11, which is equal to the anglea in the portions other than the relief groove portion 18 c, graduallydecreases from the initial position of the relief groove portion 18 c,namely, the position at which the groove-section joint angle φ is equalto −φa (=γ1), toward the innermost side.

With the ball contact angle thus reduced in the relief groove portion 18c, 22 c as compared with that in the other portions of the ball groove18, 22, a load carried by each of the balls 14 located in the reliefgroove portion 18 c, 22 c during torque transmission is reduced.

As the ball contact angle of each ball groove 18, 22 is reduced asdescribed above, the required depth of the ball groove 18, 22 forensuring the ball contact point C is also reduced. Therefore, the groovedepth of the relief groove portion 18 c, 22 c can be further reduced,which leads to increased choices in respective dimensions of each ballgroove 18, 22, such as PCR dimensions and the diameter of the ball 14,and an enhanced freedom in the design of the constant velocity joint 10.

The cross-sectional shape of the relief groove portion 18 c, 22 c hasbeen described in detail. Next, the set ranges of the relief grooveportions 18 c, 22 c in the ball grooves 18, 22 will be described indetail with reference to FIGS. 9A through 9D.

As described above, in the relief groove portion 18 c, 22 c, clearancesbetween the ball 14 and the side walls of the ball groove 18, 22 asmeasured in the circumferential direction of the inner race 11 or theouter race 13 are expanded. If all of the six balls 14 are located inthe ranges of the relief groove portions 18 c, 22 c, the inner race 11and the outer race 13 are permitted to rotate relative to each other,and cannot perform their constant-velocity rotary motion withsufficiently high reliability.

To constantly ensure the constant-velocity rotation of the inner race 11and the outer race 13, it is desirable to locate at least three of theballs 14 in the portions of the ball grooves 18, 22 other than therelief groove portions 18 c, 22 c, no matter what joint angle is, i.e.,no matter what rotational phase is established between the inner andouter races 11, 13.

FIG. 9A shows changes in the position of each ball 14 (i.e., thegroove-section joint angle φ of the ball groove 18, 22 at which eachball 14 is located) in relation to the rotational phase of the innerrace 11 and the outer race 13 when the constant velocity joint 10 ofthis embodiment having six balls 14 is rotated with the joint angle θbeing equal to the maximum angle Θmax.

In the constant velocity joint 10 of this embodiment, four or more balls14 are always present in the range in which the groove-section jointangle φ is between γ1 and γ2 even when the joint angle θ is equal to themaximum angle θmax, as shown in FIG. 9A. In this embodiment, therefore,the relief groove portions 18 c, 22 c are set to the range in which thegroove-section joint angle φ is equal to or larger than γ2 and the rangein which the groove-section joint angle φ is equal to or smaller thanγ1, respectively, so that three or more balls 14 are always present inthe portions of the ball groves 18, 22 other than the relief grooveportions 18 c, 22 c.

The set ranges of the relief groove portions required for ensuringconstant velocity rotation of the inner race 11 and the outer race 13vary with the construction or arrangement of the constant velocityjoint, including the number of balls and the maximum joint angle.

FIG. 9B-9D show changes in the position of each ball in relation to therotational phase of the inner race and the outer race when constantvelocity joints having seven, eight and nine balls, respectively, rotatein a condition in which the joint angle is set to the maximum angle.

As shown in FIG. 9B illustrating the case where the constant velocityjoint has seven balls, three or more balls are always present in therange in which the groove-section joint angle φ is between ξ1 and ξ2irrespective of the rotational phase, even when the joint angle θ isequal to the maximum angle θmax. In this example, therefore, the reliefgroove portions are set to the ranges in which the groove-section jointangle φ is equal to or smaller than ξ1 and the range in which thegroove-section joint angle φ is equal to or larger than ξ2,respectively, so that the inner race and the outer race ensureconstant-velocity rotary motion.

Similarly, in the constant velocity joint having eight balls, three ormore balls can be always located in the portions of the ball groovesother than the relief groove portions if the relief groove portions areset to be outside of the range of η1 to η2, as shown in FIG. 9C. Also,in the constant velocity joint having nine balls, three or more ballscan be always located in the portions of the ball grooves other than therelief groove portions if the relief groove portions are set to beoutside of the range of λ1 and λ2, as shown in FIG. 9D.

The constant velocity joint 10 of the embodiment as described aboveyields the following effects.

(1) In the present embodiment, the innermost portions 18 c, 22 c of theball grooves 18, 22 formed in the outer spherical surface 17 of theinner race 11 and the inner spherical surface 21 of the outer race 13are formed with relief profiles such that clearances between the ball 14and the side walls of the ball grooves 18, 22 in the circumferentialdirection are expanded or increased. The relief groove portions 18 c, 22c having the relief profiles and relatively small groove depths arearranged to receive the ball 14 only when the joint angle θ is large. Inoperation of the constant velocity joint 10, a load carried by each ball14 received in the relief groove portions 18 c, 22 c is reduced duringtorque transmission. Therefore, the constant velocity joint 10 exhibitsimproved durability, even if the relief groove portions 18 c, 22 c haverelatively small groove depths.

(2) Since a reduced load is applied to the side walls of the reliefgroove portions 18 c, 22 c of the ball grooves 18, 22 during torquetransmission, the required groove depth of the relief groove portionscan be set to a smaller value than those of conventional counterparts.The reduction in the groove depth leads to increased ranges of choicesin the PCR of each ball groove 18, 22 and the ball diameter or size, andalso leads to an enhanced freedom in the design of the constant velocityjoint 10. Consequently, the size and weight of the constant velocityjoint 10 can be advantageously reduced.

(3) In the present embodiment, the ranges of the relief groove portions18 c, 22 c are set so that three or more balls 14 are always located onthe portions of the ball grooves 18, 22 other than the relief grooveportions 18 c, 22 c, irrespective of the joint angle and the rotationalphase of the inner race 11 and the outer race 13. With this arrangement,the rotational phase of the inner race 11 and the outer race 13 is heldby at least three balls 14, thus assuring constant-velocity rotarymotion of the constant velocity joint 10.

(4) In the present embodiment, the relief groove portions 18 c, 22 c areformed by setting the ball contact angles in these portions 18 c, 22 cto be smaller than those in the other portions of the ball grooves 18,22. With the relief groove portions 18 c, 22 c thus formed, a loadcarried by the ball 14 located in the relief groove portions 18 c, 22 cduring torque transmission can be appropriately reduced. In addition,the groove depth required for ensuring contact points with the ball 14can be reduced in the relief groove portions 18 c, 22, which results ina further enhanced freedom in the design of the constant velocity joint10.

The above-described embodiment may be modified as follows.

Initially, a modified example in which the profiles of the ball groovesin the relief groove portions are modified will be described. Thecross-sectional shape of the ball grooves 18, 22 may be changed as shownin, for example, FIG. 6C, so as to increase the clearances in the reliefgroove portions 18 c, 22 c.

In the example of FIG. 6C, the PCR of the ball groove 22 in the reliefgroove portion 22 c is made longer than that in the other portions, sothat the PCR clearance CL2 in the relief groove portion 22 c is madelarger than the above-indicated predetermined length CL1 (CL2>CL1). Byincreasing the PCR clearance between the ball grooves 18, 22 in thismanner, clearances between the ball 14 and the side walls of the ballgrooves 18, 22 can be increased.

For example, the relief groove portions 18 c, 22 c may be formed bysetting the PCR of each of the ball grooves 18, 22, as shown in FIG. 8.In the example of FIG. 8, the PCR of the ball groove 22 of the outerrace 13 is gradually increased from the start position of the reliefgroove portion 22 c (at which the groove-section joint angle φ is equalto φa) toward the innermost side, so that the PCR clearance graduallyincreases from the above-indicated predetermined length CL1.

Similarly, the PCR of the ball groove 18 of the inner race 11 isgradually reduced from the start position of the relief groove portion18 c (at which the groove-section joint angle φ is equal to −φa) towardthe innermost side, so that the PCR clearance gradually increases fromthe above-indicated predetermined length CL1.

In the above arrangement in which the PCR clearances in the reliefgroove portions 18 c, 22 c are made larger than those in the otherportions of the ball grooves 18, 22, the ball 14 between the ballgrooves 18, 22 are more loosely held by the side walls of the grooves18, 22, whereby a reduced load is applied to the ball 14 located in therelief groove portions 18 c, 22 c during torque transmission between theinner race 11 and the outer race 13. If the PCR clearance is furtherincreased to a certain extent, the load applied to the ball 14 in therelief groove portions 18 c, 22 c during torque transmission can be madezero, namely, the ball 14 located in the relief groove portions 18 c, 22c can be made completely free from torque transmission.

It is to be understood that the manner of changing the ball contactangle in the set ranges of the relief groove portions 18 c, 22 c and themanner of changing the PCR of each of the ball grooves 18, 22 are notlimited to those as shown in FIG. 7 and FIG. 8, but may be suitablychanged.

It is also possible to form the relief groove portions 18 c, 22 c byreducing the contact angle in each of the ball grooves 18, 22 as well asincreasing the PCR clearance of the ball grooves 18, 22.

The relief groove portions may also be formed by a method other than thereduction of the ball contact angle, the increase of the PCR clearance,and a combination thereof. In short, the above-described effects of theillustrated embodiment or equivalent or similar effects can be obtainedprovided that the ball grooves are formed such that the load applied toeach of the balls 14 in the relief groove portions is made smaller thanthat applied to each of the balls 14 in the other portions of the ballgrooves.

The manner of setting the ranges of the relief groove portions 18 c, 22c is not limited to that of the illustrated embodiment, but may bechanged as desired. Examples of the set positions of the relief grooveportions will be listed below.

(a) A region of the ball groove 18 of the outer spherical surface 17 ofthe inner race 11 in which the groove-section joint angle φ is equal toor larger than angle 4 a and a region of the ball groove 22 of the innerspherical surface 21 of the outer race 13 in which the groove-sectionjoint angle φ is equal to or smaller than an angle −φa are set to therelief groove portions

(b) A region of the ball groove 18 of the outer spherical surface 17 ofthe inner race 11 in which the groove-section joint angle φ is equal toor larger than angle φa and a region of the ball groove 18 in which thegroove-section joint angle φ is equal to or smaller than −φa are set tothe relief groove portions.

(c) A region of the ball groove 22 of the inner spherical surface 21 ofthe outer race 13 in which the groove-section joint angle 4 is equal toor larger than angle φa and a region of the ball groove 22 in which thegroove-section joint angle it is equal to or smaller than angle −φa areset to the relief groove portions.

(d) Regions of the ball groove 18 of the outer spherical surface 17 ofthe inner race 11 and regions of the ball grooves 22 of the innerspherical surface 21 of the outer race 13 in which the groove-sectionjoint angle φ is equal to or larger than angle φa and in which thegroove-section joint angle φ is equal to or smaller than angle −φa areset to the relief groove portions.

In any of the modified examples as described above, the load applied toeach of the balls 14 located in the relief groove portions can beadvantageously reduced. In addition, if the relief groove portions areset as in the modified examples (b), (c) above, the shape of only one ofthe inner race 11 and the outer race 13 needs to be changed, thusassuring improved durability of the constant velocity joint 10.

Needless to say, the relief groove portions may be set in other mannersthan those as described above. In short, the relief groove portions maybe provided at portions of the ball grooves which are difficult toachieve sufficient mechanical strength, such as those portions having areduced groove depth due to restrictions on the profiles of, forexample, the outer spherical surface 17 of the inner race 11, the innerspherical surface 21 of the outer race 13, and the ball grooves 18, 22.With the relief groove portions thus provided, the durability of theconstant velocity joint can be improved without incurring increases inthe size or weight.

The illustrated embodiments may also be modified as follows.

The relief groove portions may be formed such that no load is applied tothe ball 14 during torque transmission, namely, no torque transmissionis carried out by the ball 14, when it is located in a part of therelief groove portion or the entire region thereof. Such a relief grooveportion in which no torque transmission occurs may be formed byincreasing a clearance in the circumferential direction to a certainextent or more.

In the illustrated embodiment, the relief groove portions 18 c, 22 c areset such that three or more balls 14 are always located in the regionsof the grooves 18, 22 other than the relief groove portions 18 c, 22 c,irrespective of the joint angle e and the rotational phase. However, therelief groove portions 18 c, 22 c may be determined otherwise. In thiscase, too, the effects other than (3) as indicated above may beobtained.

In the illustrated embodiment, the invention is applied to a constantvelocity joint of UF (undercut free) type. However, the invention isequally applicable to other types of constant velocity joints usingballs provided that the joint is constructed such that a plurality ofballs are held between the inner race and the outer race, and such thattorque is transmitted through contact points of the balls.

1. A constant velocity joint comprising: an outer race having aplurality of ball grooves formed in an inner spherical surface thereof,the ball grooves extending in a direction of a rotation axis of theouter race, an inner race having a plurality of ball grooves formed inan outer spherical surface thereof, the ball grooves extending in adirection of a rotation axis of the inner race and being paired with theball grooves of the outer race; and a plurality of balls disposedbetween the ball grooves of the outer race and the ball grooves of theinner race, wherein: the ball grooves of at least one of the outer raceand the inner race are partially formed with relief profiles to providea plurality of relief groove portions such that a load applied to eachof the balls located in the relief groove portions during torquetransmission between the inner race and the outer race is smaller than aload applied to each of the balls located in the other portions of theball grooves, wherein the ball grooves of the outer race only includeportions of the outer race in which the plurality of balls travel whenan angle formed by the rotation axis of the inner race and the rotationaxis of the outer race is less than or equal to a maximum operatingangle of the constant velocity joint.
 2. A constant velocity jointaccording to claim 1, wherein the relief groove portions are determinedso that at least one of the balls exists in the relief groove portionsonly when the angle formed by the rotation axis of the inner race andthe rotation axis of the outer race is equal to or larger than apredetermined value.
 3. A constant velocity joint according to claim 1,wherein the relief groove portions are provided at portions of the ballgrooves having a reduced depth as compared with those of the othergroove portions.
 4. A constant velocity joint according to claim 1,wherein the relief groove portions are provided in the ball grooves ofthe outer race, and are located close to an innermost side of the outerrace remote from an open end thereof as viewed in the direction of therotation axis of the outer race.
 5. A constant velocity joint accordingto claim 1, wherein the relief groove portions are provided in the ballgrooves of the inner race, and are located close to an innermost side ofthe outer race remote from an open end thereof as viewed in thedirection of the rotation axis of the inner race.
 6. A constant velocityjoint according to claim 1, wherein the relief groove portions areformed so that three or more balls are always present in portions of theball grooves other than the relief groove portions, irrespective of anangle formed by the rotation axis of the inner race and the rotationaxis of the outer race and changes in a rotational phase of the rotationaxes of the inner and outer races.
 7. A constant velocity jointaccording to claim 1, wherein the relief profiles of the relief grooveportions are formed by setting contact points between each of the ballsand an inner wall of the corresponding ball groove to be closer to acenter of the ball groove, as compared with contact points in the otherportions of the ball grooves.
 8. A constant velocity joint according toclaim 1, wherein the relief profiles of the relief groove portions areformed by setting a difference between a radius of curvature of the ballgrooves of the inner race and a radius of curvature of the ball groovesof the outer race in the relief groove portions to be larger than thatin the other portions of the ball grooves.
 9. A constant velocity jointaccording to claim 1, wherein at least a part of the ball grooves isprovided by a straight groove that extends substantially straight in thedirection of the rotation axis of a corresponding one of the inner andouter races.
 10. A constant velocity joint according to claim 1, whereinsubstantially no torque is transmitted via the balls located in therelief groove portions.
 11. A constant velocity joint according to claim1, wherein at least one ball groove of the inner race includes anarcuate groove portion extending along an arc having a center ofcurvature on the rotational axis of the inner race and a straight grooveportion that extends substantially parallel to the direction of therotational axis of the inner race.
 12. A constant velocity jointaccording to claim 1, wherein at least one ball groove of the outer raceincludes an arcuate groove portion extending along an arc having acenter of curvature on the rotational axis of the outer race and astraight groove portion that extends substantially parallel to thedirection of the rotational axis of the outer race.
 13. A constantvelocity joint according to claim 1, wherein a first ball contact angleof at least one ball groove is set at a predetermined angle over anentire region of a portion of the at least one ball groove other thanthe relief groove portion of the at least one ball groove.
 14. Aconstant velocity joint according to claim 13, wherein the relief grooveportion of the at least one ball groove includes a second ball contactangle that is less than the first ball contact angle.
 15. A constantvelocity joint according to claim 14, wherein a ball contact angle ofthe relief groove portion of the at least one ball groove graduallydecreases from a first ball contact angle to a second ball contactangle.
 16. A constant velocity joint according to claim 1, wherein theball grooves of the outer race extend from an innermost side of theouter race to an outermost side of the outer race, wherein the innermostside of the outer race corresponds to a position occupied by aninnermost one of the plurality of balls when the angle formed by therotation axis of the inner race and the rotation axis of the outer raceis the maximum operating angle of the constant velocity joint, andwherein the outermost side of the outer race corresponds to a positionoccupied by an outermost one of the plurality of balls when the angleformed by the rotation axis of the inner race and the rotation axis ofthe outer race is the maximum operating angle of the constant velocityjoint.
 17. A constant velocity joint according to claim 1, wherein theball grooves of the outer race extend from an open end portion of theouter race to an inner end portion of the outer race remote from theopen end portion.
 18. A constant velocity joint according to claim 1,wherein the ball grooves of the inner race only include portions of theinner race in which the plurality of balls travel when the angle formedby the rotation axis of the inner race and the rotation axis of theouter race is less than or equal to the maximum operating angle of theconstant velocity joint.
 19. A constant velocity joint comprising: anouter race having a plurality of ball grooves formed in an innerspherical surface thereof, the ball grooves extending in a direction ofa rotation axis of the outer race, an inner race having a plurality ofball grooves formed in an outer spherical surface thereof, the ballgrooves extending in a direction of a rotation axis of the inner raceand being paired with the ball grooves of the outer race; and aplurality of balls disposed between the ball grooves of the outer raceand the ball grooves of the inner race, wherein: the ball grooves of atleast one of the outer race and the inner race are partially formed withrelief profiles to provide a plurality of relief groove portions suchthat a clearance between each of the balls located in the relief grooveportions and inner walls of the ball grooves as viewed in acircumferential direction is larger than a clearance between each of theballs located in the other portions of the ball grooves and the innerwalls of the ball grooves, wherein the ball grooves of the outer raceonly include portions of the outer race in which the plurality of ballstravel when an angle formed by the rotation axis of the inner race andthe rotation axis of the outer race is less than or equal to a maximumoperating angle of the constant velocity joint.
 20. A constant velocityjoint according to claim 19, wherein the relief groove portions aredetermined so that at least one of the balls exists in the relief grooveportions only when the angle formed by the rotation axis of the innerrace and the rotation axis of the outer race is equal to or larger thana predetermined value.
 21. A constant velocity joint according to claim19, wherein the relief groove portions are provided at portions of theball grooves having a reduced depth as compared with those of the othergroove portions.
 22. A constant velocity joint according to claim 19,wherein the relief groove portions are provided in the ball grooves ofthe outer race, and are located close to an innermost side of the outerrace remote from an open end thereof as viewed in the direction of therotation axis of the outer race.
 23. A constant velocity joint accordingto claim 19, wherein the relief groove portions are provided in the ballgrooves of the inner race, and are located close to an innermost side ofthe outer race remote from an open end thereof as viewed in thedirection of the rotation axis of the inner race.
 24. A constantvelocity joint according to claim 19, wherein the relief groove portionsare formed so that three or more balls are always present in portions ofthe ball grooves other than the relief groove portions, irrespective ofan angle formed by the rotation axis of the inner race and the rotationaxis of the outer race and changes in a rotational phase of the rotationaxes of the inner and outer races.
 25. A constant velocity jointaccording to claim 19, wherein the relief profiles of the relief grooveportions are formed by setting contact points between each of the ballsand an inner wall of the corresponding ball groove to be closer to acenter of the ball groove, as compared with contact points in the otherportions of the ball grooves.
 26. A constant velocity joint according toclaim 19, wherein the relief profiles of the relief groove portions areformed by setting a difference between a radius of curvature of the ballgrooves of the inner race and a radius of curvature of the ball groovesof the outer race in the relief groove portions to be larger than thatin the other portions of the ball grooves.
 27. A constant velocity jointaccording to claim 19, wherein at least a part of the ball grooves isprovided by a straight groove that extends substantially straight in thedirection of the rotation axis of a corresponding one of the inner andouter races.
 28. A constant velocity joint according to claim 19,wherein substantially no torque is transmitted via the balls located inthe relief groove portions.
 29. A constant velocity joint according toclaim 19, wherein at least one ball groove of the outer race includes anarcuate groove portion extending along an arc having a center ofcurvature on the rotational axis of the outer race and a straight grooveportion that extends substantially parallel to the direction of therotational axis of the outer race.
 30. A constant velocity jointaccording to claim 19, wherein at least one ball groove of the innerrace includes an arcuate groove portion extending along an arc having acenter of curvature on the rotational axis of the inner race and astraight groove portion that extends substantially parallel to thedirection of the rotational axis of the inner race.
 31. A constantvelocity joint according to claim 19, wherein a first ball contact angleof at least one ball groove is set at a predetermined angle over anentire region of a portion of the at least one ball groove other thanthe relief groove portion of the at least one ball groove.
 32. Aconstant velocity joint according to claim 31, wherein the relief grooveportion of the at least one ball groove includes a second ball contactangle that is less than the first ball contact angle.
 33. A constantvelocity joint according to claim 32, wherein a ball contact angle ofthe relief groove portion of the at least one ball groove graduallydecreases from a first ball contact angle to a second ball contactangle.
 34. A constant velocity joint according to claim 19, wherein theball grooves of the outer race extend from an innermost side of theouter race to an outermost side of the outer race, wherein the innermostside of the outer race corresponds to a position occupied by aninnermost one of the plurality of balls when the angle formed by therotation axis of the inner race and the rotation axis of the outer raceis the maximum operating angle of the constant velocity joint, andwherein the outermost side of the outer race corresponds to a positionoccupied by an outermost one of the plurality of balls when the angleformed by the rotation axis of the inner race and the rotation axis ofthe outer race is the maximum operating angle of the constant velocityjoint.
 35. A constant velocity joint according to claim 19, wherein theball grooves of the outer race extend from an open end portion of theouter race to an inner end portion of the outer race remote from theopen end portion.
 36. A constant velocity joint according to claim 19,wherein the ball grooves of the inner race only include portions of theinner race in which the plurality of balls travel when the angle formedby the rotation axis of the inner race and the rotation axis of theouter race is less than or equal to the maximum operating angle of theconstant velocity joint.